FOURIER OPTICS 355
distance between points where the signal has increased from minimum by 10% and 90% of
the difference [173]. This so-called 90-10 width is a useful single number to characterize the
system. Levels other than 10% and 90% may be used as well. However, examination of the
complete ESF offers further information such as the presence of sidelobes and other details that
may be of interest. Again, the ESF can be measured in different directions to ensure that the
system meets the desired specications. The ESF can, in principle, be differentiated to obtain
the LSF. We must be careful, however, because taking the derivative increases the effect of
noise, just as we saw in using deconvolution to obtain the PSF. Effectively, we are subtracting ...